LONDON—The co-pilot who flew a Germanwings jetliner into a French mountainside appeared to have rehearsed the deadly descent on the plane’s previous flight that same morning, French authorities said.

Andreas Lubitz, who investigators say deliberately crashed Flight 9525 after locking the pilot out of the cockpit, briefly set the plane’s altitude to as low as 100 feet several times during its outbound flight from Düsseldorf, Germany, to Barcelona on March 24, the Bureau d’Enquêtes et d’Analyses, France’s air-safety agency,...